[PATCH v3 01/13] mm/huge_memory: simplify vma_is_specal_huge()

Lorenzo Stoakes (Oracle)
2026-03-20T18:07:39Z

This function is confused - it overloads the term 'special' yet again,
checks for DAX but in many cases the code explicitly excludes DAX before
invoking the predicate.

It also unnecessarily checks for vma->vm_file - this has to be present for
a driver to have set VMA_MIXEDMAP_BIT or VMA_PFNMAP_BIT.

In fact, a far simpler form of this is to reverse the DAX predicate and
return false if DAX is set.

This makes sense from the point of view of 'special' as in
vm_normal_page(), as DAX actually does potentially have retrievable
folios.

Also there's no need to have this in mm.h so move it to huge_memory.c.

No functional change intended.

Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
include/linux/huge_mm.h | 4 ++--
include/linux/mm.h | 16 ----------------
mm/huge_memory.c | 30 +++++++++++++++++++++++-------
3 files changed, 25 insertions(+), 25 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 8d801ed378db..af726f0aa30d 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -83,7 +83,7 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
* file is never split and the MAX_PAGECACHE_ORDER limit does not apply to
* it. Same to PFNMAPs where there's neither page* nor pagecache.
*/
-#define THP_ORDERS_ALL_SPECIAL \
+#define THP_ORDERS_ALL_SPECIAL_DAX \
(BIT(PMD_ORDER) | BIT(PUD_ORDER))
#define THP_ORDERS_ALL_FILE_DEFAULT \
((BIT(MAX_PAGECACHE_ORDER + 1) - 1) & ~BIT(0))
@@ -92,7 +92,7 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
* Mask of all large folio orders supported for THP.
*/
#define THP_ORDERS_ALL \
- (THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_SPECIAL | THP_ORDERS_ALL_FILE_DEFAULT)
+ (THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_SPECIAL_DAX | THP_ORDERS_ALL_FILE_DEFAULT)

enum tva_type {
TVA_SMAPS, /* Exposing "THPeligible:" in smaps. */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8aadf115278e..6b07ee99b38b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -5078,22 +5078,6 @@ long copy_folio_from_user(struct folio *dst_folio,
const void __user *usr_src,
bool allow_pagefault);

-/**
- * vma_is_special_huge - Are transhuge page-table entries considered special?
- * @vma: Pointer to the struct vm_area_struct to consider
- *
- * Whether transhuge page-table entries are considered "special" following
- * the definition in vm_normal_page().
- *
- * Return: true if transhuge page-table entries should be considered special,
- * false otherwise.
- */
-static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
-{
- return vma_is_dax(vma) || (vma->vm_file &&
- (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)));
-}
-
#endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */

#if MAX_NUMNODES > 1
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e90d08db219d..2775309b317a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -103,6 +103,14 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
}

+/* If returns true, we are unable to access the VMA's folios. */
+static bool vma_is_special_huge(const struct vm_area_struct *vma)
+{
+ if (vma_is_dax(vma))
+ return false;
+ return vma_test_any(vma, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT);
+}
+
unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
vm_flags_t vm_flags,
enum tva_type type,
@@ -116,8 +124,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
/* Check the intersection of requested and supported orders. */
if (vma_is_anonymous(vma))
supported_orders = THP_ORDERS_ALL_ANON;
- else if (vma_is_special_huge(vma))
- supported_orders = THP_ORDERS_ALL_SPECIAL;
+ else if (vma_is_dax(vma) || vma_is_special_huge(vma))
+ supported_orders = THP_ORDERS_ALL_SPECIAL_DAX;
else
supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;

@@ -2338,7 +2346,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
tlb->fullmm);
arch_check_zapped_pmd(vma, orig_pmd);
tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
- if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
+ if (vma_is_special_huge(vma)) {
if (arch_needs_pgtable_deposit())
zap_deposited_table(tlb->mm, pmd);
spin_unlock(ptl);
@@ -2840,7 +2848,7 @@ int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
orig_pud = pudp_huge_get_and_clear_full(vma, addr, pud, tlb->fullmm);
arch_check_zapped_pud(vma, orig_pud);
tlb_remove_pud_tlb_entry(tlb, pud, addr);
- if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
+ if (vma_is_special_huge(vma)) {
spin_unlock(ptl);
/* No zero page support yet */
} else {
@@ -2991,7 +2999,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
*/
if (arch_needs_pgtable_deposit())
zap_deposited_table(mm, pmd);
- if (!vma_is_dax(vma) && vma_is_special_huge(vma))
+ if (vma_is_special_huge(vma))
return;
if (unlikely(pmd_is_migration_entry(old_pmd))) {
const softleaf_t old_entry = softleaf_from_pmd(old_pmd);
@@ -4517,8 +4525,16 @@ static void split_huge_pages_all(void)

static inline bool vma_not_suitable_for_thp_split(struct vm_area_struct *vma)
{
- return vma_is_special_huge(vma) || (vma->vm_flags & VM_IO) ||
- is_vm_hugetlb_page(vma);
+ if (vma_is_dax(vma))
+ return true;
+ if (vma_is_special_huge(vma))
+ return true;
+ if (vma_test(vma, VMA_IO_BIT))
+ return true;
+ if (is_vm_hugetlb_page(vma))
+ return true;
+
+ return false;
}

static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
--
2.53.0